New N test helps to get the most out of manures

There’s no doubt manure is valuable, but difficulties and uncertainties in assessing its nutrient content may put many farmers off using manure nutrients to replace bought-in fertiliser.

Not so for Philip Huxtable of 3,600ha JSR Farms in East Yorkshire, who can clearly see its value. He applies slurry to his wheat in the spring through a 24m dribble bar boom and later uses a tractor-mounted N-sensor to detect areas in need of a top-up with bagged N fertiliser.

“Last season, we saved £195,000 using slurry rather than buying in nitrogen and made a net benefit of £227,000 when the extra yield was accounted for. I know we are fortunate in having what I refer to as a fertiliser factory – our 3,500 sows plus their progeny – to supply much of the nitrogen, phosphate and potash required by the cereal, oilseed rape and potato crops, but any amount of manure is worth exploiting.”

Slurry typically supplies 90kgN/ha of the 210kgN/ha required to maximise wheat yields. Over the past eight years, average yield has increased by 0.6t/ha.

This season, Mr Huxtable has valued what he calls the “liquid gold” at £142/ha, based on current fertiliser prices. The “solid gold” manure applied to stubbles in the autumn is worth a similar amount.

He is aware of the variability in NPK levels in manure, and knows through regular testing that his products can be quite different in some years to the typical values given in DEFRA’s fertiliser recommendation guide, RB209.

“It’s critical we test our farm-produced muck so we can manage it to maximise yield potential while minimising lodging. We feel lucky to be one of the first to use the new NIRS manure analysis service and we will evaluate it this year alongside other nutrient tests. We hope it will allow further fine tuning.”

Developed during a DEFRA Link project involving 16 research and industry partners, the test promises to be more accurate, quicker and cheaper than the main route of wet chemistry. It uses near infra-red reflectance spectroscopy (NIRS) to measure dry matter, total N and readily available ammonium-N, phosphorus, potassium, sulphur and magnesium.

It is being offered from this month as a service by one of the project partners, Eurofins. It is currently available for cattle and pig manures and slurries, plus water company biosolids, and there are plans to develop it for poultry manures.

Sampling

ADAS principal soil scientist, Ken Smith, believes there’s nothing wrong with conventional wet chemistry analysis as a useful guide to what manure contains. However, he is excited by the NIRS test because it works with a larger sample, which is particularly important for solid manures, and is less likely to give a misleading result.

“Eurofins’ new test uses a large part of the recommended 2kg sample supplied by the farmer rather than a tiny fraction of it, typically 5g for wet chemistry. Each 2kg sample is scanned by the equipment three times and averaged. And there’s less chance of ammonium-N loss – the crop available form – during the process.”

But as with any analysis, the result will be reliable only if sampling is representative of the manure store, he warns. A detailed study of sampling procedure using the NIRS test confirmed the benefits of the existing guidelines for careful sampling.

For solids, he recommends taking 10 sub-samples of 1kg, though the results of a sampling study suggested 15-20 may be required for untidy, variable heaps constructed over several months. Use a clean bucket to mix the sub-samples before putting a composite sub-sample of 2kg into a labelled plastic bag, he says.

With slurry, mixing before sampling is best, otherwise dip at least 10 times into the lagoon, mix and then sub-sample to obtain a one litre composite sample, recommends Dr Smith.